FXO is an RJ11 interface which is providing a connection to a PSTN Central Office (CO) or to a PBX interface.
A FXO is considered as a telephony line. We can then find 2 types of signaling :
- Loop-start signaling : this is a type of supervisory signaling that allows indication of on-hook and off-hook states. But this signaling is vulnerable to a condition knows as glare that involves 2 endpoints seizing a trunk a the same time.
- Ground-start signaling : Same as loop-start signaling but this time we suppress the glare effect on these lines.